The development of the organ of Corti is a tightly regulated process, in which a small prosensory domain of the cochlear duct is sorted into various types of hair cells and support cells. While it known that intercellular signals, like Notch and FGF, are required at several steps in cochlear development, their precise roles and relationship at the stage of sensory cell specification are not clear. In the past year, we have found that FGF20 is necessary for sensory cell specification. Inhibition of FGF20 or inhibition of the FGF receptors leads to a loss in Atoh1 expression. We have also found that inhibition of Notch signaling in explant cultures at early stages of cochlear development leads to a block in sensory cell development. These findings have led to a working model in which the prosensory Notch signal leads to the expression of Fgf20, which then activates Atoh1 in the sensory cells. In this submission, we propose several lines of investigation aimed at testing specific hypotheses about the molecular mechanisms of sensory cell specification. Our proposal has the following specific aims: 1. To determine whether the prosensory action of Notch is mediated by FGF20; 2. To determine whether the action of Notch on FGF20 expression is direct; 3. To determine whether Ets domain transcription factors are necessary and/or sufficient for sensory cell specification. A better understanding of the molecular pathways regulating normal development will be critical for rational strategies for hair cell replacement and regeneration in adult onset deafness.